1. Field of the Invention
This invention relates to invasive medical devices. More particularly, this invention relates to ablation of tissue using such devices.
2. Description of the Related Art
Ablation of body tissue using electrical energy is known in the art. The ablation is typically performed by applying alternating currents, for example radiofrequency energy, to the electrodes, at a sufficient power to destroy target tissue. Typically, the electrodes are mounted on the distal tip of a catheter, which is inserted into a subject. The distal tip may be tracked in a number of different ways known in the art, for example by measuring magnetic fields generated at the distal tip by coils external to the subject.
A known difficulty in the use of radiofrequency energy for cardiac tissue ablation is controlling local heating of tissue. There are tradeoffs between the desire to create a sufficiently large lesion to effectively ablate an abnormal tissue focus, or block an aberrant conduction pattern, and the undesirable effects of excessive local heating. If the radiofrequency device creates too small a lesion, then the medical procedure could be less effective, or could require too much time. On the other hand, if tissues are heated excessively then there could be local charring effects, coagulum, and or explosive steam pops due to overheating. Such overheated areas can develop high impedance, and may form a functional barrier to the passage of heat. The use of slower heating provides better control of the ablation, but unduly prolongs the procedure.
Steam pops or microbubble formation can occur during RF ablation when tissue temperatures exceed 100 C. While tactile and audible cues are used for their detection, background lab noise and catheter movement may confound identification. Steam pops are particularly hazardous. For example, during RF ablation, steam pops caused by tissue overheating may result in cardiac perforation. The present disclosure deals with recognition of impending steam pops in time to take corrective action, e.g., controlling the power output of the ablator.
Previous attempts to recognize steam pops include the use of phonocardiography. For example, the document Detection of microbubble formation during radiofrequency ablation using phonocardiography, Kotini et al., EP Europace Volume 8, Issue 5 Pp. 333-335 proposes detection of characteristic signatures prior to steam pops using a computer-based phonocardiography system.
In another approach, the document Steam Pop Prediction and Detection During Radiofrequency Ablation, Holmes et al., Circulation 2011 124:A13330 describes an Electrical Coupling Index (ECI), in which resistive and reactive impedance between the ablation catheter and tissue. The ECI was displayed as a continuous waveform during ablation. A steam pop prediction algorithm is said to have predicted 92% of all steam pops at least 3 seconds before they occurred. The negative predictive value of the steam pop prediction algorithm is stated as 98%.
U.S. Pat. No. 8,147,484 to Lieber et al. discloses real-time optical measurements of tissue reflection spectral characteristics while performing ablation. The technique involves the radiation of tissue and recapturing of light from the tissue to monitor changes in the reflected optical intensity as an indicator of steam formation in the tissue for prevention of steam pop. Observation is made to determine whether measured reflectance spectral intensity (MRSI) increases in a specified time period followed by a decrease at a specified rate in the MRSI. If there is a decrease in the MRSI within a specified time and at a specified rate, then formation of a steam pocket is inferred.